1. Field of the Invention
The invention relates to microlithographic projection exposure apparatus, such as those used for the production of large-scale integrated electrical circuits and other microstructured components. The invention relates in particular to a method by which the optical polarization properties of such apparatus can be evaluated and improved.
2. Description of Related Art
Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers to a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of diffracting structures, which is arranged on a reticle, is then imaged onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than one, such projection objectives are also often referred to as reduction objectives.
After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the reticle. The remaining photoresist is then removed from the remaining parts of the layer. This process is repeated until all the layers have been applied to the wafer.
One of the essential aims in the development of microlithographic projection exposure apparatus is to be able to generate structures with smaller and smaller dimensions on the wafer, so as to thereby increase the integration density of the components to be produced. By using a wide variety of measures, it is now possible to generate structures on the wafer whose dimensions are less than the wavelength of the projection light being used.
Particular importance is in this case attached to the polarization state of the projection light arriving on the photoresist. This is because the polarization state has a direct effect on the contrast which can be achieved, and therefore the minimum size of the structures to be generated. This is related to the fact that the reticle diffracts the transmitted projection light into different diffraction orders, which interfere in the image plane and thereby produce the image of the reticle. The interference phenomena between different diffraction orders are commensurately more pronounced as the polarization states of the interfering diffraction orders are similar. Full constructive or destructive interference between two diffraction orders is possibly only if they are polarized in the same way.
The polarization dependency of the contrast becomes particularly noticeable in projection objectives with high numerical apertures, such as those which are possible in immersion objectives. In the case of projection light which is polarized perpendicularly to the incidence plane (s-polarization), the interference phenomena of two plane waves which lie in the same incidence plane are independent of the angle at which the different diffraction orders arrive on the photoresist.
In the case of projection light polarized parallel to the incidence plane (p-polarization), however, different diffraction orders can no longer interfere fully since the diffraction orders have different polarization directions. The interference phenomena are then commensurately weaker as the angles with respect to the optical axis, at which the diffraction orders arrive on the photoresist, are large. If the propagation directions of the interfering diffraction orders in the resist form a right angle, then the polarization vectors are mutually perpendicular and interference no longer takes place at all. If the included angle is more than 90° then although the contrast increases again, the regions of constructive and destructive interference which correspond to regions of maximum and minimum intensity nevertheless become interchanged relative to the case with an included angle of less than 90°. The differences in the interference behavior between s-polarized and p-polarized projection light are therefore particularly important in projection objectives with a high numerical aperture.
For this reason, attempts are made to configure the optical subsystems of the projection exposure apparatus, i.e. the illumination system and the projection objective, so that the projection light in the image plane is optimally polarized. A homogeneous s-polarization is often sought for the aforementioned reasons; under certain circumstances, for example in the case of projection objectives with a small numerical aperture, it may be more expedient to have other polarization states such as circularly polarized or completely unpolarized projection light.
The polarization distribution which is set up in the image plane depends on a multiplicity of factors. The polarization state of the projection light, with which the illumination system of the projection exposure apparatus illuminates the reticle, naturally has a very significant effect. Besides this, every optical interface alters to a greater or lesser extent the polarization state of light which does not arrive perpendicularly on the interface. Many of the optical materials used in projection objectives are furthermore intrinsically birefringent or have induced birefringence, and therefore also modify the polarization state of the projection light. Filters and other manipulators, by which the polarization state can be deliberately affected and corrected, are furthermore known.
Optimum configuration and correction of the projection exposure apparatus in terms of optical polarization requires that the effects and imaging errors associated with the polarization state can be described and evaluated in a way which is straightforward, physically as informative as possible but nevertheless precise.